For optimum operation, it is normally required that a multicylinder engine obtain equal performance from each of its cylinders. A common cause of unequal performance among the cylinders is asymmetry in the intake manifold causing unequal or dissimilar distribution of charges to the cylinders.
For example, engines in which fuel and recirculated exhaust gas (EGR) are mixed completely with air upstream of the manifold runners can suffer from maldistribution of the total charge, although fuel, air and EGR may be delivered in equal ratios to each cylinder. In other cases in which mixing persists as the charge flows through the manifold or when fuel or EGR are added downstream of the manifold, any asymmetry in the shape or arrangement of the runners can cause differences in charge composition among cylinders Even if all cylinders receive charges of the same composition and in the same amount, the characteristics of flow into the cylinders can differ because of differences in the path taken by the charge. These variations can result in differences in combustion rate between the cylinders. Finally, if the intake runners in a tuned intake system differ in lengths, then different cylinders will have different volumetric charging efficiencies as a function of engine speed.
Modern tuned intake manifolds for inline engines commonly consist of parallel runners connecting the intake ports of each cylinder to a single plenum. Thus, the plenum can be as long as the engine. Because the plenum volume is limited by packaging and weight constraints and by the requirements of intake system tuning, the plenum shape approaches that of a long cylinder or other oblong body. With air, and possibly EGR and/or fuel, entering at one point in the plenum, the potential for maldistribution along the length of the plenum is high.
If the manifold does not distribute the components of the charge adequately, then corrective dams or flow deflectors must be added experimentally until the design is satisfactory. Alternatively, the runners can be made to enter the plenum close together for a more compact plenum shape. However, unless the runners are symmetrically disposed and thorough mixing of air with fuel and EGR in the plenum can be guaranteed, maldistribution may still occur. In addition, the runners may now have different lengths and curvatures, introducing new sources for cylinder-to-cylinder charging differences.